The present invention relates to a method for heat treatment of a silicon wafer, in particular, such a heat treatment method which can improve microroughness on a silicon wafer surface and can suppress generation of slip dislocations and heavy metal contamination, as well as a silicon wafer of high quality useful for semiconductor devices, which is obtained by the heat treatment.
In recent years, use of higher integration degree of MOS-LSI has made gate oxide films increasingly thinner. And in order to obtain reliability of such thin oxide films, quality of silicon wafers as substrates is considered important. Inter alia, microroughness on surfaces of wafers attracts much attention.
More precisely, with the use of higher integration degree of MOS structure transistors, it has become necessary to improve the mobility of carriers (electrons and holes) immediately under the oxide layer in the MOS structure. Moreover, with the use of increasingly higher driving frequency of CPU (central processing unit), higher writing and reading speeds of memories are of course required, and therefore improvement of the carrier mobility has become an important research subject.
Further, it has become clear that microroughness on wafer surfaces is closely related to performance and reliability of devices as a factor greatly affecting the electric characteristics such as the oxide dielectric breakdown voltage and the mobility of carriers (see Shinya Yamakawa et. al., J. Appl. Phys. 79, 911, 1996).
As a method for reducing the microroughness of wafer surface, there has been known, for example, a method which comprises applying an electric current to the wafer surface using a special apparatus such as an ultra high vacuum apparatus (see Ando et al., Extended Abstracts (The 56th Autumn Meeting, 1995); The Japan Society of Applied Physics, 27p-ZV-13, 1995). However, it takes long time to obtain ultra high vacuum, and it also takes long time to return the vacuum to atmospheric pressure. This causes a problem that one must always pay attention to adhesion of particles during those process steps.
Further, as a method for improving the microroughness, there is already the proposal of Japanese Patent Application No. 10-176693, and sufficiently small microroughness has been obtained. In this technique, however, heat treatment is performed at a high temperature by using a rapid heating and rapid cooling apparatus under a reducing atmosphere with multiple steps. Therefore, the process becomes complicated and problems may arise in productivity, durability of the apparatus, generation of slip dislocations, heavy metal contamination and so forth.
Usual silicon wafers have a SiO2 layer called natural oxide film on their surfaces. In order to remove this film, hydrogen annealing at a high temperature of about 1200xc2x0 C. is required. However, such hydrogen annealing has problems such as the generation of slip dislocations and the heavy metal contamination from the inside of the system. Therefore, it is necessary to simultaneously achieve a lower temperature of the whole process and improvement of microroughness. It is considered that lowering temperature of the whole process will become still more important because relative mechanical strength of wafers will become weaker as a larger diameter of wafers is used in future.
The present invention has been accomplished in view of the aforementioned problems, and its object is to provide a method for heat treatment of silicon wafers under a reducing atmosphere utilizing a rapid heating/rapid cooling apparatus (also referred to as RTA (Rapid Thermal Annealer) apparatus hereinafter), which can, in particular, afford further reduced microroughness on silicon wafer surfaces by heat treatment at a lower temperature compared with conventional techniques, improve electric characteristics such as the oxide dielectric breakdown voltage and the mobility of carriers, and suppress generation of slip dislocations and heavy metal contamination, as well as utilize the advantages proper to the rapid heating/rapid cooling apparatus, for example, improved yield and productivity, low cost and so forth.
In order to achieve the aforementioned object, the present invention provides a method for heat treatment of a silicon wafer under a reducing atmosphere using a rapid heating/rapid cooling apparatus, wherein a natural oxide film on a silicon wafer surface is removed, and then the silicon wafer is subjected to a heat treatment under an atmosphere of 100% hydrogen or a mixed gas atmosphere of argon and/or nitrogen containing 10% or more of hydrogen using a rapid heating/rapid cooling apparatus.
If, in a method for heat treatment of a silicon wafer under a reducing atmosphere using a rapid heating/rapid cooling apparatus, the silicon wafer is subjected to a heat treatment under an atmosphere of 100% hydrogen or a mixed gas atmosphere of argon and/or nitrogen containing 10% or more of hydrogen using a rapid heating/rapid cooling apparatus after a natural oxide film on a silicon wafer surface is removed as described above, microroughness on the wafer surface can surely be reduced while maintaining the state of the wafer that the natural oxide film is removed. At the same time, crystal defects existing on the wafer surface can also be eliminated, and thus there can be obtained a silicon wafer useful for semiconductor devices with extremely few defects and excellent electric characteristics.
In this case, the aforementioned heat treatment can be performed at a temperature of 950 to 1150xc2x0 C. for 1-300 seconds.
By subjecting a wafer from which a natural oxide film is removed to a heat treatment at a lower temperature compared with conventional techniques for a short period of time using a rapid heating/rapid cooling apparatus, the microroughness can markedly be reduced. Moreover, since the heat treatment temperature is relatively lowered, the temperature of the whole process is also lowered. Therefore, the generation of slip dislocations is substantially eliminated, the problem of heavy metal contamination is also solved, and the yield is improved. Furthermore, since time and thermal energy required for temperature increase and decrease can also be reduced, improvement of productivity and cost reduction can also be contemplated.
The present invention also relates to the method for heat treatment of a silicon wafer, wherein the natural oxide film is removed by hydrofluoric acid treatment.
SiO2 constituting the natural oxide film can easily and surely be dissolved and removed by immersion in hydrofluoric acid (HF) or gaseous phase treatment utilizing HF gas, and original flatness, microroughness and so forth of a mirror-finished wafer before the treatment are not substantially affected adversely.
Furthermore, because the microroughness of the wafer surface is improved by the method for heat treatment of the present invention, there can be obtained a silicon wafer of extremely high quality useful for semiconductor devices, which has improved electric characteristics such as the oxide dielectric breakdown voltage and the mobility of carriers, and is substantially free from generation of slip dislocations and heavy metal contamination.
In particular, there can be obtained a silicon wafer having a microroughness of 1.00 nm or less in terms of Pxe2x88x92V value and 0.12 nm or less in terms of RMS value as determined by an atomic force microscope for a 2 xcexcm square.
As explained above in detail, by subjecting a silicon wafer from which a natural oxide film is removed to a heat treatment at a relatively low temperature under a reducing atmosphere of 100% hydrogen or a mixed gas atmosphere of argon and/or nitrogen containing 10% or more of hydrogen using a rapid heating/rapid cooling apparatus, microroughness on the wafer surface can markedly be reduced. As a result, there can be obtained a silicon wafer of excellent crystallinity, which has excellent electric characteristics such as the oxide dielectric breakdown voltage and the mobility of carriers, and is substantially free from generation of slip dislocations and heavy metal contamination. At the same time, improvement of yield and productivity can be contemplated, and cost reduction can be achieved.